The present invention relates to apparatuses for treating materials with microwave energy and, more particularly, to energy traps for preventing the escape of microwave energy into the environment from microwave ovens.
The processing of materials by means of microwave energy radiated within an enclosure has become widespread. Microwave oven systems enjoy the advantages of efficiency, fast processing times, low cost, and more uniform heating in the case of lossy dielectric materials. For the purposes of the present discussion, the term microwave refers to electromagnetic energy having wavelengths in the range between one meter and one millimeter or frequencies in excess of 300 megahertz.
To provide for continuous operations, many industrial microwave ovens have been provided with conveyors which carry the material to be treated into, through, and out of the enclosure of the oven. A serious drawback of these continuous flow process systems has been that the access openings to the enclosure can leak considerable microwave energy into the surrounding environment. Such leakage may result in medical detriment to persons in the area and possibly interference with nearby communications services. On account of these hazards, there is a need to provide efficient devices which will reduce the aforementioned leakage.
Various solutions have been proposed to deal with the problem of microwave energy escaping through the access ports of conveyorized microwave ovens. U.S. Pat. No. 2,868,939 to Pound discloses the use of lossy dielectric energy absorbing panels disposed in a vestibule adjoining an access port. The panels are coated with a resistive material and oriented in the plane, or planes, of maximum electric field excitation in the vestibule. This structure is not sufficiently effective to reduce energy leakage to the desired level. U.S. Pat. No. 3,048,686 to Schmidt discloses the use of multiple, adjustable quarterwave blocking filters in tunnels adjacent to the oven access ports. As an extra feature, Schmidt also discloses the use of slots in the tunnel walls apparently for reflection of radiation back into the oven enclosure. This device is complicated and expensive to assemble. U.S. Pat. No. 3,624,335 to Dench discloses slotlike discontinuities in the walls of access tunnels adjoining the entrance and exit ports of a microwave oven. These discontinuities function to reflect microwave energy back into the main oven enclosure. Additionally, Dench disclosed the use of sheets of energy absorbent material enclosing the discontinuities apparently for the purpose of assuring attenuation of escaping energy not adequately reflected by the slots. This structure is effective but still allows unacceptable amounts of microwave energy leakage. U.S. Pat. No. 3,858,022 to Smith discloses an attenuation channel having spaced plates forming passages containing microwave absorbing material and an intermediate section wherein liquid lossy to microwaves is circulated into access with escaping microwave energy. This apparatus is effective but is of such a complicated structure that its cost is prohibitive so as to preclude general use.